The broad, long-term objective of this research is to reduce the prevalence of marijuana use and related harm during the transition to university, through the development of efficacious prevention and intervention techniques for first-year college students. Marijuana use is the most commonly used illicit substance among college students, with up to 35% of students reporting use within the past year. The primary aim of the current project is to document the efficacy of an internet-based personalized normative feedback (PNF) intervention, compared to an assessment-only condition, in preventing marijuana initiation among never using first-year college students and in reducing use and related negative consequences of currently using students. A secondary aim is to evaluate several potential moderators and mediators of intervention efficacy (e.g., readiness to change, psychiatric symptoms, alcohol consumption, and demographic variables). The specific aims of this research will be accomplished in three phases. Phase 1 will consist of a measure development component. Phase 2 will be to develop the PNF intervention for marijuana use adapted from a protocol previously used for reducing high-risk drinking. Phase 3 will employ a longitudinal design to evaluate the efficacy of the internet-based PNF program for two groups of incoming college-freshmen: marijuana nonusers (n=300) and marijuana users (n=300), identified through brief screening of a random sample of UW incoming freshmen (N=2670) and based on lifetime and past-year use. Students in each group, user and non-user, will be randomized into an intervention group or an assessment-only control group. Students in the intervention groups will immediately receive PNF upon completion of the baseline survey. All students will be assessed 3-, 6-and 9-months post-baseline. This research will fill important knowledge gaps regarding the use of normative feedback in preventing and reducing marijuana use and will serve as a prototype for implementing low-cost internet based personalized normative feedback interventions.